Recent years saw development in high-speed, large-capacity data communications and cellular communications. With respect to multilayer substrates having integrated circuits, this development led to not only size-reduction and increased density but also investigations as to use of signals having frequencies in a high-frequency band ranging from, for example, several ten megahertz to several hundred gigahertz. While ceramic compositions are used in these multilayer substrates, it is desirable that the ceramic compositions be made of a material (high-frequency band material) compatible with signals in the high-frequency band.
Typically, alumina (Al2O3) has been primarily used as the ceramic composition for the high-frequency band. As the density of the integrated circuits increases, there has been developed a process of making a multilayer substrate including an integrated circuit, the process including stacking a plurality of green sheets composed of unsintered Al2O3, each green sheet having a conductor paste that contains a material for metal wiring applied by printing, and then simultaneously baking the green sheets and the conductor paste. Since Al2O3 sinters at a temperature as high as 1,500° C. to 1,600° C., a high melting point-metal, such as tungsten or molybdenum, which can withstand such a high temperature, has been required as the material for the metal wiring of the integrated circuit.
The multilayer substrate has a problem in that it requires a large amount of energy since the sintering temperature is high, thereby increasing the manufacturing cost. Since the thermal expansion coefficient of Al2O3 is larger than that of the IC chip, such as a silicon chip, in the integrated circuit, the multilayer substrate may suffer from cracks depending on the operating temperature of the multilayer substrate. Furthermore, since the relative dielectric constant of Al2O3 is large, the rate of signal propagation in the integrated circuit has been low. Since the specific resistance of a high melting point-metal, such as tungsten or molybdenum, is large compared to that of Cu or Ag, which is suitable as a material for the metal wiring, the conductor loss due to the resistance of the metal wiring itself has also been large.
In view of the above, various ceramic compositions each in which a filler is incorporated in a glass composition have been developed as the material for multilayer substrates. Multilayer substrates using such ceramic compositions can be sintered at a temperature lower than that when Al2O3 is used. Thus, it becomes possible to simultaneously sinter the ceramic compositions and the material for metal wiring, such as Cu or Ag, having a smaller specific resistance. Furthermore, since the filler is contained in the glass composition, the change in shape of the ceramic composition can be reduced, and the strength of the ceramic composition can be increased.
For example, Japanese Examined Patent Application Publication No. 3-53269 described an example of such a ceramic composition, prepared by sintering a mixture of a CaO—SiO2—Al2O3—B2O3 glass composition and 50 to 35 mass % of Al2O3 as a filler at 800° C. to 1,000° C. Japanese Patent No. 3277169 discloses a ceramic composition containing 0 to 10 mol % of Al2O3 as a filler and a glass composition including 50 to 67 mol % of B2O3, 2 to 3 mol % of an oxide of an alkali metal element, 20 to 50 mol % of an oxide of an alkaline earth metal element, and 2 to 15 mol % of an oxide of a rare earth element. Japanese Unexamined Patent Application Publication No. 9-315855 discloses a ceramic composition containing an oxide of a rare earth element, Al2O3, CaO, and TiO2, and in which the compounding ratio of these components is limited within a particular range.
The performances required for the ceramic composition for the high-frequency band include a low dielectric loss tanδ in the high-frequency band and a small absolute value of the temperature coefficient τf of the resonant frequency.
In other words, the loss in the course of signal propagation in the high-frequency band is preferably as small as possible. Thus, it is desirable that the dielectric loss tanδ of the ceramic composition in the high-frequency band be small, i.e., that the Q value (1/tan δ) be large. Moreover, in order to yield stable performance from the ceramic composition serving as a dielectric member despite a temperature change, it is desirable that the absolute value of the temperature coefficient τf of the resonant frequency be small, i.e., that the temperature dependence of the resonant frequency be low.